1. Field of the Invention
The present invention relates to an amplitude and phase demodulator circuit for signals with very low modulation index. The invention is related to the first stages of receiver circuit RX which have the purpose of optimally demodulating and operating a first amplification of a demodulated signal. More particularly, the invention is suitable to be applied to communication systems between a "transponder" 2 and a base station 1, as schematically depicted in FIG. 1.
2. Discussion of the Related Art
The base station includes a transmitter TX that generates a carrier whose frequency is generally 125 KHz. This in turn generates, by means of the tuned circuit C1-L1, a magnetic field in the coil L1 working as an antenna.
A second inductor L2 is placed in this magnetic field, working as an antenna, connected to a second tuning capacitor C2 and to an electronic circuit. This one includes a secret code and a circuit which is able to modulate the voltage of the resonant circuit L2-C2 with a sequence of high and low values, which correspond to the sequence of binary digits (bit) composing the secret code itself.
If the inductor L2 is placed to a distance sufficiently close to L1, but without the need of electromechanical contacts between the two, a magnetic coupling M between L1 and L2 appears, which is sufficient to generate at the ends of L2-C2 a voltage for supplying the transponder electronic circuit 3. This supply system which needs neither batteries nor contacts may be called "remote supply".
The transponder internal electronic circuit is supplied by the AC voltage at the terminals of L2-C2, which is by itself properly rectified and smoothed, and is able to transmit the code included in its memory. To do this the electronic circuit absorbs either a high or a low current from the resonant circuit L2-C2, in accordance with the binary value, respectively low or high, to be transmitted.
This current consumption modulation is applied to L2-C2 and propagates to the resonant circuit L1-C1, attenuated by the low coupling coefficient associated with the mutual inductance M.
The Vrx signal on the resonant circuit L1-C1 is provided by the TX transmitted carrier, hence with a rather high level, and by a modulating component as a result of that explained above.
The Vrx signal is sent to the input of an RX reception circuit which has the purpose of carrying out a demodulation and hence of reconstructing the data of the transponder memory. This data represents the secret code which is subsequently interpreted by a microcomputer .mu.C.
The RX circuit is generally able to process signals with voltages not any higher than 5V, for reasons of economy of the materials with which it is built. Hence its input signal Vrx must be first attenuated if it is not included within such limits. Though, this means that the modulating signal will typically be 5V/1000=5 mV, but actually RX will have to guarantee good performance with modulating signals as low as 1 mV.
The Vrx signal is typically amplitude modulated, but because of misalignments among the resonance frequencies of the tuned circuits L1-C1 and L2-C2 and the excitation frequency coming out from TX, phase modulation components may appear as well.
FIG. 2 shows the amplitude and phase Bode plots of the resonator L1-C versus the excitation frequency that in our case happens to be the carrier output from TX.
The plots show two curves both for amplitude and phase, which correspond to the two cases of current consumption at the transponder side.
If the resonator is well tuned with respect to the carrier frequency (case 1) the two levels cause amplitude variation but no phase variation. But, if there is a small misalignment between resonant and excitation frequencies (case 2), a phase modulation appears together with the amplitude modulation. Finally, if the misalignment becomes wider (case 3), then the amplitude modulation disappears and the phase modulation remains.
FIGS. 3, 4, and 5 show the waveforms versus time for cases 1, 2, and 3 respectively.
The sensitivity of a receiver substantially depends on its equivalent input noise, to which the first amplifier stage of the receiver chain contributes, together with all those stages between this one and the receiver input which do not provide meaningful amplification, as for example a demodulator can be.
FIGS. 6A, 6B and 6C show an example of prior art in which the input signal Vrx immediately becomes amplified, then demodulated. However, the demodulation circuit must work at high voltages, therefore it has the inconvenience of having low performance as it must be simple, or else it would be too expensive. This solution can only demodulate the amplitude but not the phase, hence case 3 of FIG. 2 and FIG. 5 cannot be handled.
FIGS. 7A, 7B and 7C show a second example of prior art in which the Vrx signal becomes first demodulated with a multiplication with a square wave (at the mixer node 4) which is synchronous (SYNC) with the transmitted carrier (obtaining a Va signal), then it becomes smoothed by a low-pass filter 5 that eliminates the residual carrier frequency component (125 KHz typical), but that doesn't affect the base band signal (100 Hz-5 KHz typical).
The resulting Vb signal may have a DC component as high as the amplitude of the Vrx input signal, hence no amplification is possible before this point. A subsequent high pass filter 6, with a cut off frequency that is lower than the base band lower limit (100 Hz typical), may eliminate the high DC component and simultaneously amplify and obtain a useful signal Vout.
The SYNC signal must have a proper phase with respect to the carrier as shown in FIG. 8: accordingly, a maximum difference between the two levels, high and low, of the demodulated signal is reached. Often the search for the optimum demodulation phase is carried out by an algorithm that is implemented by a microprocessor.
This kind of solution is widely used, though it has the drawback of having a certain number of elements that, located before the high pass filter with first amplifying stage, contribute to the equivalent input noise by limiting the input sensitivity.
The main sources of noise are:
the phase jitter of the SYNC signal, with the maximum effect right in the case of perfect alignment of the two antennas with the carrier frequency, where the phase component modulation is absent; PA1 the mixer, which is a simple voltage follower with a switching gain between +1 and -1 according to the commanding SYNC signal; PA1 the low pass filter. PA1 amplifier means adapted to amplify a modulated signal coming from a transmitter, said modulated signal being composed by a carrier and by a modulating component, means adapted to cancel said carrier from said modulated signal; said means adapted to cancel the carrier receiving as an input the output signal of said amplifier means and a sync signal coming from said transmitter, the output signal of said amplifier means being, delivered to receiver means. PA1 subtracting from a modulated signal, output from a transmitter, a carrier signal generated by said means adapted to cancel the carrier, obtaining a pre-processed signal; PA1 amplifying said pre-processed signal; and PA1 feeding back to said means adapted to cancel the carrier a signal obtained by said amplifying step to derive an output signal to be delivered to receiver means.
FIGS. 9A and 9B show a third example of a prior art circuit in which the input signal Vrx is demodulated by means of sampling with a signal (SYNC) which is synchronous with the transmitted carrier and suitably phased with respect to such carrier.
The sampled signal Va is then stored by a block 7 which uses the same SYNC signal and which has a low-pass filtering feature: accordingly, the signal is smoothed and the derived signal, Vb, is then pass-band filtered in a filter 8 (with a gain Av) to cancel the DC component of the signal to obtain the Vout signal.
The advantage of this system is represented by the fact that the sampling takes into account the only portion of the Vrx signal in which the difference between the two logic levels (high and low) is maximum, i.e., the information content is higher.
Hence a better signal to noise ratio is achieved. Moreover, the blocks before the first gain stage Av, that carry out the sample and hold action, may be simply realized for example with capacitors and switches, thus contributing to a low equivalent input noise.
A further advantage is that the phase jitter of the synchronism signal (SYNC) is not converted to noise when Vrx only shows the amplitude modulation component (thanks to a good frequency alignment of the carrier with the two antennas) and the sampling instant occurs at the maximum or minimum peak of the Vrx sinusoid.
It must be noted that the synchronism phase of the demodulation has an optimum point that needs to be located with an identical algorithm to that for the case of FIG. 7B.
FIG. 10 shows a practical embodiment of the above described principle with reference to FIG. 9.
The synchronism is made by two properly phased signals with the carrier that, when they are active (high), close the switches S1 and S2. The switch S1 and the capacitor 15 generate a Va signal corresponding to a level shifted Vrx signal, so that, when S1 is closed, the Va instantaneous value equals the reference voltage (zero in this example) that is connected to the low side of S1.
The subsequent value assumed by Va when S2 is closed, is transferred to the capacitor 16 with a low pass filter action, due to elements 17 and 16, that eliminates the carrier frequency residuals.
The obtained Vb signal is then processed by the band-pass filter with gain Av that eliminates the DC level, amplifies and further eliminates carrier residuals and noise at frequencies outside the signal base band.
This solution offers good performance with respect to sensitivity thanks to the simplicity and to the low noise level of the demodulator, which uses switches and capacitors.
However it has the drawback of not being easily realizable in an integrated form. In fact, the dimensions of the components do not allow this and, in addition, the voltage range of Va and Vb is twice as much compared with that of Vrx. Hence the Vrx range cannot be optimized with respect to the supply voltage, which is normally 5V.